It is well known that various derivatives of 2,5-dimercapto-1,3,4-thiadiazole are useful lubricant, grease and fuel additives for inhibition of copper activity, i.e., corrosion or staining. Many patents have been issued on various derivatives of 2,5-dimercapto-1,3,4-thiadiazole. Amongst various derivatives of 2,5-dimercapto-1,3,4-thiadiazole, the bisulfides are preferred as corrosion inhibitors/metal passivator additives for lubricants, greases and fuels. These disulfides react with large quantities of sulfur or active sulfur compounds to give compounds, which are non-corrosive to metals, especially copper and silver. Their capacity to react with sulfur is large, as 2,5-bis (dodecyldithio)-1,3,4-thiadiazole is able to react with 16 gram atoms of sulphur and the resulting product is a non-corrosive in modified EMD silver strip test. Thus 2,5-dimercapto thiadiazole disulfides have the ability to act as “sulfur sink”.
U.S. Pat. No. 3,087,932 discloses a method of making 2,5-bis( alkyldisulphide)-1,3,4-thiadiazole. This process involves the reaction of hydrogen peroxide with 2,5-dimercapto-1,3,4-thiadiazole or its alkyl metal salt and alkyl mercaptan.
U.S. Pat. No. 4,097,387 describes metal deactivators derived from 2,5-dimercapto-1,3,4-thiadiazole by reaction with sulfur halide and an olefin to form an intermediate, which is then treated with sodium or potassium salt of thiadiazole. More recently, U.S. Pat. No. 4,487,706 describes the preparation of metal deactivator by reacting olefin, sulfur dichloride and 2,5-dimercapto-1,3,4-thiadiazole in a single step.
Other method of making these disulphides of thiadiazole involves reaction of chloromercaptan with 2,5-dimercapto-1,3,4-thiadiazole or formation of 2,5-dimercapto thiadiazole dichloride and subsequent reaction with mercaptans. This latter reaction is specially preferred in view of the high temperature stability of the dichlorides than alkyl sulfenyl chlorides. These methods have been reviewed by Ellis K Fields in Industrial and Engineering Chemistry, Vol. 49, 1957, p. 1361 to 1364; which is incorporated herein as reference.
As can be seen from the above description of the prior-art, the metal deactivator/corrosive inhibiting additives from 2,5-dimercapto-1,3,4-thiadiazole have been prepared either by reaction of thiadiazole with alkyl mercaptans or by using olefins and sulfur dichloride.
The disadvantages of the above processes are:                Alkyl mercaptans are expensive material and the cost of corrosion inhibitor/metal deactivator additives produced by these procedures becomes very high.        Methane thiol and ethane thiol have low boiling point of 6° C. and 35° C. respectively and therefore difficult to handle commercially in bulk.        Methane thiol and ethane thiol have a very strong odour.        
In U.S. Pat. No. 4,487,706, during the reaction of thiadiazole with sulfur dichloride and olefin, hydrogen chloride vapours are evolved and the additives contain residual chlorine by introduction of the HCl gas into olefinic unsaturation. This process evolves corrosive HCl gas and the product contains higher amount of chlorine and hence is not advantageous.
Our U.S. Pat. No. 6,362,137 provides a process for preparing a corrosion inhibitor/metal passivator additive for lubricant, grease and fuel applications from waste refinery streams, comprising reacting 2,5-dimercapto-1,3,4-thiadiazole either in powder form or in solution form in polar organic solvent, with disulphide oil obtained from Merox extraction/caustic wash of the refinery streams.
In order to get good yields, a redox reagent like hydrogen peroxide was used. The subsequent reaction with disulfide oil was carried out in the same vessel and therefore some amount of hydrogen peroxide always remained in the mixture. Since the reaction mixture was refluxed in the latter part of the process, a small amount of sulphones resulted from the oxidation of disulfides. Though these sulphones were subsequently removed during the step of hexane extraction, an additional step was introduced. Thus, by the two step synthesis, we reported the preparation of 2,5-bis(alkyldithio)-1,3,4-thiadiazoles by using the disulfides from waste refinery streams and having an average molecular composition of C5H12S2 to C12H26S2.